1. Field of the Invention
The present invention relates to a vibratory flow meter and zero check method.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things.
When there is no flow through the flow meter, all points along the conduit oscillate due to an applied driver force with identical phase or a small initial fixed phase offset which can be corrected. As material begins to flow, Coriolis forces cause each point along the conduit to have a different phase. The phase on the inlet side of the conduit lags the driver, while the phase on the outlet side of the conduit leads the driver. Pickoff sensors coupled to the conduit(s) to produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pickoff sensors are processed to determine the phase difference between the pickoff sensors. The phase difference between two pickoff sensor signals is proportional to the mass flow rate of material through the conduit(s).
Coriolis mass flow meters calculate mass flow rate from a time delay measurement where time delay arises from the Coriolis effect and is directly proportionally to the mass flow rate. For an ideal Coriolis mass flow meter (one that is completely symmetric from its inlet to its outlet and is undamped) measuring time delay is all that is needed to accurately determine mass flow rate. However, Coriolis mass flow meters are inevitably non-symmetric and are subject to structural and viscous damping. As a result, under no flow conditions a small amount of time delay may be present. This time delay is measured and subtracted from the time delay induced by the Coriolis effect to obtain a zero time delay.
It is a problem that the time delay of a Coriolis flow meter at zero flow may change over time. Changes in the zero-flow time difference can result in an erroneous flow rate measurement.
Coriolis flow meters often require zeroing, such as during an initial calibration, during operation, or both. Zeroing a Coriolis mass flow meter at the factory comprises filling the meter with a desired, known flow material under strictly controlled conditions, establishing zero flow of the flow material, making sure that the fluid is stable, such as ensuring that there are no entrained gases in the flow material if the flow material is a liquid, vibrating the meter assembly and taking a number of samples and obtaining multiple zero-flow time difference values, calculating an average zero-flow time difference (or other representative time difference value), and storing a calibration zero-flow time difference (Δt0) in the Coriolis mass flow meter.
In operation, the zero-flow time difference (Δt0) may be used in the Coriolis flow meter for generating mass flow measurements. Mass flow is determined as:mass flow=FCF*(Δt−Δt0)  (1)
The FCF term is a flow calibration factor representative of physical characteristics of the flow meter. The (Δt) term is the current, measured time difference between pickoff signals. The (Δt0) term is the stored zero-flow time difference calibration value. The (Δt) term comprises a measurement signal that is generated during operation of a Coriolis flow meter.
In the prior art, a zero-flow calibration may be initiated in an operational environment by a user. One drawback in the prior art is that such a user-initiated zero-flow calibration process may be done whether it is needed or not. A previously generated and stored zero-flow time difference (Δt0) value may be accurate enough to generate good measurement values.
Another drawback in the prior art is that during a zeroing operation in the field, it may not be possible to strictly control all environmental conditions. The fluid in the meter to be zeroed will generally not be a calibration fluid provided just for the operation. Problems with the fluid, such as entrained gas in a liquid flow material, may disturb the time difference (Δt) readings so that the calculated zero-flow time difference (Δt0) is not representative of the true average. As a result, the meter may be zeroed incorrectly, introducing errors.
Yet another drawback is that the user performs a zeroing process without knowing whether the current zero-flow time difference is accurate or inaccurate. Re-zeroing a flow meter when it has an accurate zero-flow time difference could result in a new zero-flow time difference value that is similar to or even less accurate than the previous value.
Yet another drawback in the prior art is that the user is left to assume that the newly-produced zero value is accurate (and more accurate than the previous value). To assess accuracy of (Δt0), flow meter users often zero the meter multiple times and compare the produced (Δt0) values. This is cumbersome, expensive, and time consuming, and puts too much expectation on flow meter users to understand how the zeroing process works.